Fuel cells are electrochemical systems that directly produce electrical energy from two chemical compounds, typically a fuel and an oxidizer. FIG. 1 shows a typical fuel cell system in which an electrolyte 104 separates a fuel and an oxidizer. Electrolyte 104 serves as a proton exchange membrane (PEM), which is a hydrogen ion conductor but an electronic insulator. Hydrogen atoms are catalyzed from Hydrogen gas or a hydrocarbon source at the anode and disassociate from their electrons. The electrons flow through an electrode assembly not shown, through an external load 116, and back to the cathode 112. The Hydrogen ions are conducted through the PEM 104 and combine at the cathode 112 with electrons and oxygen to form water or steam, a waste product. The electrical current passing from cathode 112 to anode 120 through the external load 116 provides useful electrical energyA more detailed description of fuel cell operation is provided in a Fuel Cell Handbook, by Appleby, A. J. and Foulkes, F. R., and published by Van Nostrand Reinhold Co, New York, 1989.
In the illustrated system, electrolyte 104 is ideally made as thin as possible. Thin electrolytes are desirable because thinner structures are better ionic conductors and offer reduced electrical resistance. Typically, Ionic conductance is inversely proportional to thickness while electrical resistance is approximately proportional to thickness. High electrical resistance across the electrolyte increases power losses.
However, making electrolyte 104 thin increases fabrication difficulties and increases the probability of electrolyte failure. First, a thin electrolyte may not be effective at separating fuel and oxidizer. Fuel that diffuses through the electrolyte along with its electrons decreases cell efficiency because the electrons do not pass through the external circuit to provide useful energy. This situation is called fuel crossover. Fuel crossover oxidizes at the cathode 112 and generates heat. This is one limitation of using thin Nafion-based membranes with methanol fuel. Second, many fuel cell membrane technologies use soft or brittle materials. Thin electrolytes made from such materials are often mechanically unstable. If the membrane leaks or ruptures and allows bulk mixing of fuel and oxidizer, the cell fails and the device may explode or burn as catalytic materials in the anode and cathode permit runaway exothermic reactions. Fuel cell designers must balance safety and fuel crossover (which suggest thicker electrolytes) and ion conduction efficiency (which suggests thinner electrolytes). In order to solve the problem of structurally weak electrolytes, U.S. Pat. No. 4,863,813 by Dyer et al. eliminates a separating electrolyte and combines the oxidizer and the fuel in a common region 204 as shown in FIG. 2. In order to prevent a runaway fuel-oxidizer reaction, catalysts that enhance the reaction are shielded from the reacting species. To shield the catalysts, the Dyer patent teaches including the catalyst in electrode compositions and using selectively permeable electrodes. Thus, for example, the anode may be permeable to fuel but not to oxidizer. Designs of such selectively permeable electrodes are further described in Taylor et al (U.S. Pat. No. 5,102,750) and Ellgen et al (U.S. Pat. No. 5,162,166). However, the fabrication of such selectively permeable electrodes is difficult and the resulting constraints on electrode design results in non-optimal performance.
Thus an improved system of forming an electrolyte structure that maintains separation of the fuel and oxidizer yet avoids the tradeoff between mechanical robustness of the electrolyte is needed. For electrolytes in which fuel crossover is not significant, a method of mechanically stiffening a thinner electrolyte would allow better ion conduction and efficiency.